Currently, semiconductor devices are hermetically sealed using gold-tin (Au—Sn) solder with gold concentrations ranging from 78% to 81% and tin concentrations ranging from 19% to 22%. This solder has been in use in semiconductor devices for more than 40 years and has a melting temperature of 280° C. See U.S. Pat. No. 4,492,842 to Koopman. Semiconductor devices and a myriad of other electronic devices are subsequently soldered onto a printed circuit board using tin-lead solders as a post-sealing solder with melting temperatures of 183° C. to 195° C. The process temperature of these post-sealing solders is typically 35° C. to 50° C. above the melting temperature (i.e. 218° C. to 245° C.). Superheat is required to flow solder onto the leads of the semiconductor device and the copper circuitry of the printed circuit board. Processing the semiconductor devices at temperatures up to 245° C. poses no problem since the gold-tin sealing solder will not re-melt until 280° C.
The prior art lead-free solders that are replacing the tin-lead solders melt at 217° C. and require processing temperatures approaching 270° C. These processing temperatures are dangerously close to the 280° C. melting temperature of the Au—Sn solder used to seal the device. One option is to abandon the tin-based solders and use lead-based solders that contain about 85% lead. Such lead solders have a melting point of about 300° C. The use of lead-based solders is, however, generally avoided by the electronics industry.
As lead continues to be removed from consumer electronics, the need grows for a higher temperature solder to be developed for semiconductor die attach and hermetic sealing of these semiconductor devices. There is a need to replace conventional Au—Sn and lead-based solders with a lead-free, high temperature solder. This need is met by the lead-free alloys of the present invention, whose melting temperature and hermetic sealing capabilities make them especially suitable for use as semiconductor die attach solder and as a hermetic sealing solder for semiconductor devices.